(a) Field of the Invention
The present invention relates to a system for correcting presbyopia. More particularly, the invention is concerned with a combination of two contact lenses where one is more specially adapted to correct distance vision and the other, reading vision. The invention is for example achieved by utilizing a system of two different sphero-non-spherical contact lenses. In other words, the present invention is directed to a system of visual correction enabling the simultaneous correction of presbyopia and myopia or hypermetropia, by means of two contact lenses of different geometry, and which are complementary to one another. One of these lenses is specifically placed on the dominating eye to assist distance vision while the other lens is disposed on the other eye to improve vision reading.
(b) Description of Prior Art
Presbyopia consists of a decrease of the accommodation power (convergence) of the crystalline lens, which prevents a person from having a good vision at close distance. This loss of accommodation of the crystalline lens affects everybody who reaches the age of 40 years and older. The loss of the accommodation power of the crystalline lens is progressive, less important at the start and is more noticeable as the years go by. It gradually becomes impossible to read a text without having a visual correction.
Presbyopia affects emmetropic eyes, i.e. which present a normal vision, as well as ametropic eyes, i.e. which present an anomaly of the ocular refraction, such as myopia, hypermetropia and astigmatism.
Accommodation, in the field of optometry, means a modification of the curvature of the crystalline lens of the eye providing a greater convergent power in order to project a clear image on the retina when the objects which are perceived are close.
Another term which needs to be defined in order to clarify the description of the present invention, is the word addition. It means a convergent increment to an optical correction to compensate for the loss of convergence (accommodation) of the crystalline lens. The increment is directly proportional to the importance of presbyopia. For example, a presbyope having a loss of accommodation equivalent to 2.00 diopters will need an addition of +2.00 diopters.
On the other hand, a myopic person whose correction for distance vision is -3.50 diopters and whose presbyopia is of 2.00 diopters, should have a correction of -3.50 diopters for distance vision and a correction of -1.50 diopter for reading, i.e. (-3.50)+(+2.00 addition)=-1.50.
A person suffering from hypermetropia whose distance vision correction is +3.50 diopters and whose presbyopia is +2.00 diopters, will need a correction of +3.50 diopters for distance vision and a correction of +5.50 diopters for reading vision, i.e. (+3.50)+(+2.00 addition)=+5.50.
An emmetropic person whose distance vision is normal and whose presbyopia is 2.00 diopters, will need no correction for distance vision but will require a correction of +2.00 diopters for reading vision, i.e. (0.00)+(+2.00 addition)=+2.00.
A diopter is a unit of measure of the vergency of optical systems which is equivalent to the vergency of an optical system whose focal distance is 1 meter. The symbol "+" is used for converging optical systems, i.e. which converge optical rays; the symbol "-" is used for diverging optical systems, i.e. which diverge optical rays.
When used in the present context, the term spherical defines the characteristic of a surface in which the points are generated by radii of curvature of identical lengths, originating from a single point. More specially, it represents the characteristic of an optical system which has one single focal point, i.e. meeting point of the optical rays. In general, a spherical optical system is generated by one or more spherical surfaces.
The term non-spherical indicates a surface in which the points are generated by radii of curvature of different lengths which originate from different points placed on the same axis. It is used in association with an optical system which has a plurality of focal points, i.e. converging point of the optical rays. In general, a non-spherical system is generated by one or more non-spherical surfaces.
Focus or focal point is the converging point of an optical system where the image is formed.
Another term which needs definition in the context of the present invention, is the expression dominating eye. It means the eye in which visual perception is predominant for distance vision. The other eye is considered to be the dominated eye. Finally, the term power means the dioptric power of a converging "+" or diverging "-" optical system.
It is known to use an optical additional correction called addition for the purpose of compensating for the loss of accommodation of the crystalline lens. The addition has a positive dioptric power (converging) of a value which is equivalent to the loss of the converging power of the crystalline lens. The means for visual correction for persons having presbyopia, in the form of spectacles are numerous and very efficient. However, the situation is different when the means of visual correction are contact lenses.
For correcting presbyopia with contact lenses, four different groups of lenses are available:
1. simple spherical lenses used in monovision; PA1 2. bifocal lenses; PA1 3. non-spherical lenses; PA1 4. sphero-non-spherical lenses. PA1 a first multifocal contact lens comprising on a front face thereof, a first central area having a spherical surface, the central circular area being dimensioned to cover about 65% to about 85% of a pupil of an eye of a person, and a first annular shaped area surrounding the first central area, having an non-spherical surface which has a dioptric power in the form of a progressive addition which varies from a zero addition at the inner periphery of the first annular shaped area to a maximum addition at the outer periphery thereof, the non-spherical area enabling vision at all distances including reading, PA1 the first central area incorporating a correction for distance vision, the first annular shaped non-spherical area incorporating gradual correction from distance correction to intermediate and reading corrections, PA1 the first multifocal contact lens being adjusted to adapt to the vision of a dominant eye of that person, PA1 a second multifocal lens comprising on a front face thereof a second central area having a spherical surface being dimensioned to cover about 45% to 70% of a pupil of another eye of the person, and a second annular shaped area surrounding the second central area, having a non-spherical surface which has a dioptric power in the form of a progressive reduction which varies from a maximum addition at the inner periphery thereof to a zero addition at the outer periphery thereof, the non-spherical area enabling vision at all distances including reading, PA1 the second central area incorporating a correction for reading, the second annular shaped area incorporating gradual correction from reading to intermediate and distance corrections, PA1 the second multifocal contact lens being adjusted to adapt to the vision of a dominated eye of that person.
Simple spherical lenses constitute a type of correction which utilizes two spherical lenses having a single focus. The dominating eye is corrected for distance vision and the other for reading vision. This is surely the most popular way of correcting presbyopia in spite of its limitation. The reason for the popularity of this type of adjustment resides in its great simplicity and the low cost of the lenses used. With this type of adjustment, the wearer loses his binocular vision since only one of his eyes sees at a distance and only one is used for close vision. This type of vision is, at the limit, tolerable for early presbyopic people for whom the addition is less than +1.50 diopters. For the others, the loss of binocular effect becomes intolerable. The long term negative effects on visual perception have not yet been definitely established. In addition, this type of adjustment does not provide comfortable vision for objects located at intermediate distance.
There are three types of bifocal contact lens, namely segmented lenses, concentric lenses and diffractive lenses.
A segmented lens is divided into two distinct optical zones. The upper part is for correcting distance vision, and the segment constituting the lower part is for close vision, such as reading. The two optical zones have spherical surfaces.
In order to alternately use the two zones of vision, the lens must undergo a vertical displacement relative to the pupil. For this displacement to take place, the lens should rest on the edge of the lower lid and be maintained by the latter when inclining the head in reading position. This often causes a problem as a person gets older, since the tonus of the lower lid substantially decreases which causes the lens to slide under the lid, thereby preventing an adequate displacement which is required when using the lower reading zone.
This type of geometry does not provide a comfortable vision at intermediate distances, since there are only two focuses and because the optical corrections are limited to close or distant objects. For example, a person working on a computer will have a good vision of the keyboard and of distant objects, however the screen will be blurred. It should be noted that this problem increases as presbyopia increases since the difference between the two corrections increases.
Concentric lenses are divided into two distinct optical zones, i.e. a central circular zone of spherical type surrounded by an annular zone which is also of spherical type. There are two versions of this basis geometry. One consists in providing the correction for distance vision in the central part of the lens and the correction for close vision, in the annular part. The other version consists in reversing the order of these corrections.
This type of geometry does not provide a comfortable vision at intermediate distances, since there are only two focuses and the optical corrections are limited to distance or close objects. For example, a person working on a computer will have a good vision of the keyboard and of remote objects, however vision of the screen will be blurred. It should be noted that this problem is aggravated with the progression of presbyopia since the difference between the two corrections increases.
This type of lens utilizes a simultaneous vision mode, i.e. the two zones of vision of the lens are used simultaneously and they are permanently before the pupil. The utilization of this type of lens for night vision often produces light reflections which disturb the visual perception mainly when driving a car. The abrupt passage (without transition) of the correction for distance vision to close vision produces a marked junction at the common limit of the two zones thereby resulting in a parasite diffraction effect of the light rays. It should be noted that the use of lenses with concentric focuses is limited to two lenses of the same version (identical) for a person wearing them.
Turning now to diffractive lenses, they are divided into a plurality of concentric circular zones and rely on the principle of diffraction to produce two distinct focuses: one for correcting distance vision and the other for close vision. This type of geometry does not provide comfortable vision at intermediate distances since there are only two focuses and the optical corrections are limited to remote or close objects. For example, a person working on a computer will have a good vision of the keyboard and of distant objects, however vision of the screen will be blurred. It should be noted that this problem increases with the progression of presbyopia since the difference between the two corrections widens.
This type of lens utilizes the simultaneous vision mode, i.e. all the vision zones of the lens are used simultaneously and they are permanently before the pupil.
With non-spherical lenses, the correction (dioptric power) changes gradually from the center for close vision towards the periphery for distance vision or vice versa. The front surface is of the non-spherical type which generates a non-spherical optical system.
This type of lens utilizes the mode of simultaneous vision, i.e. the different zones of vision of the lens are utilized simultaneously and they are permanently before the pupil. Because of its design, this type of lens includes a correction for close, intermediate and distance vision. However, the design involves an important limitation. The correction for close vision (addition) is not a variable parameter and is therefore fixed.
There is a pre-established relation between the correction for distance vision and the correction for close vision. It therefore becomes difficult do obtain a true correction simultaneously for distance and close vision. In practice, it is necessary to sacrifice the quality of vision at a given distance in favor of the other. This limitation becomes more and more disastrous with the increase of presbyopia which requires a difference of correction which is increasingly important between distance vision and close vision. At the same time a compromise which is more and more important is required between the qualities of distance and close visions.
The geometry with entirely non-spherical surfaces decreases the quality of the perceived image by provoking a loss of contrast. The retina simultaneously receives a plurality of images of a same object of which some are on focus (precise) and others are out of focus (unprecise). A superimposition of images which are simultaneously precise and unprecise results in a global unprecise image. The degree of inaccuracy of the image increases with the non-spherical aspect of the surface and the latter increases with a variation of intended optical correction, and consequently with the increase of the addition (increase of presbyopia).
There is a direct relationship between the diameter of the pupil and the variation of available power (dioptric power). Because the dioptric power of the lens varies gradually from the center towards the periphery, a larger pupil will utilize a greater variation of power, while a smaller pupil will utilize a smaller variation. The result, mainly for small pupils, is the impossibility of obtaining an adequate correction for distance or close vision. Moreover, variations of lighting conditions largely influence the diameter of the pupil and at the same time the visual performance with this type of lens.
Sphero-non-spherical lenses are formed of two optically distinct zones, i.e. a spherical circular central zone and a non-spherical annular zone surrounding the latter. This type of lens utilizes the mode of simultaneous vision, i.e. the different zones of vision of the lens are utilized simultaneously and they are permanently before the pupil.
This type of lens actually includes only lenses in which the correction for distance vision is in a central spherical zone surrounded by a non-spherical annular zone for intermediate and close vision. Reference is made to U.S. Pat. No. 5,125,729.
Many advantages are associated to this type of sphero-non-spherical design. The first advantage is that the corrections for distance and close vision are distinct and independent from one another. It is therefore possible to obtain, without limitation, the exact optical correction for distance and close vision without having to modify one to the detriment of the other.
The second advantage results from the fact that the central zone provides a higher concentration of images on focus (precise) to the retina and thus gives a global image with superior contrast (more precise) as compared to entirely non-spherical lenses. Another advantage is the fact that the non-spherical annular zone enables a correction of the vision at intermediate distances in addition to correcting close vision.
The fourth advantage is due to the fact that passing from a correction for distance correction to one for close vision is carried out gradually. There is therefore no abrupt separation at the junction of the two zones, which eliminates parasites rays in night vision especially when driving a car.
Finally, since the two zones are distinct, it is possible to vary their dimensions in addition to the correcting power. It is therefore possible to vary the diameter of the two zones as a function of the diameter of the pupil of the candidates and thus to improve the visual performance of the candidates for whom the diameter of the pupils is different from the average.
As a general rule, with sphero-non-spherical lenses, the quality of distance vision is very good. However, visual performance in close vision could be improved since it is derived from a zone in which the correction is generated by a non-spherical surface. This type of lens is therefore less precise with respect to the quality of the perceived image.
It will therefore be seen that none of the systems discussed above, give a completely satisfactory correction for a person in need of bifocal lenses. This is especially true of the various combinations of lenses discussed above.
There is therefore a need for lenses which maintain good distance vision, but at the same time improve intermediate and close visions. To Applicant's knowledge, this has not been achieved with the lenses of the prior art.